Electrostatic gas-scrubber and method

ABSTRACT

A method for separating dust from a gas and a gas-scrubber in which droplets of a liquid which is evaporable in the milieu prevailing in the gas are mixed with the gas, and in which the droplets ejected into the gas are electrically charged and are brought to a potential such that the surface charge density of the droplets substantially corresponds to an electric field strength equalling the brakedown field strength of the gas. The droplets thus charged and while carried by the gas are passed with the gas through a gas flow zone in which the droplets are exposed to evaporation, to cause a self-maintained discharge of the electric charge of the droplets due to a tendency of increase of field strength in the vicinity of a droplet as the droplets are diminished during evaporation. Thereby the charges carried by the droplets are transferred to the dust particles carried by the gas and thereafter the gas carrying the particles thus charged and any remaining droplets, are passed through an electric filter for separating the dust particles thus charged from the gas.

United States Patent [191 Romell 1 ELECTROSTATIC GAS-SCRUBBER AND METHOD[76] Inventor: Dag Romell, 30, Tomavagen, 223

63 Lund, Sweden 221 Filed: Sept. 30, 1970 21 Appl. No.: 76,774

[30] Foreign Application Priority Data Sept. 30, 1969 Sweden", 13410/69[52] US. Cl 55/10, 55/11, 55/107, 55/122, 55/135 [51] Int. Cl B01d 3/01[58] Field of Search 55/5, 7, 8, 9, 10, 11, 55/107, 122, 135, 136-138,120

[56] References Cited UNITED STATES PATENTS 2,024,226 12/1935 Irwin eta1 55/122 2,357,354 9/1944 Penney 55/107 2,525,347 10/1950 Gilman 55/103,41 1,025 11/1968 Marks 55/107 3,503,704 3/1970 Marks 55/8 1,130,2143/1915 Steere 55/11 1,940,198 12/1933 Wagner 55/107 FOREIGN PATENTS ORAPPLICATIONS 629,665 10/1961 Canada 55/138 1 Apr. 30, 1974 12/1918 GreatBritain 55/122 6/1920 Great Britain 55/11 ABSTRACT A method forseparating dust from a gas and a gasscrubber in which droplets of aliquid which is evaporable in the milieu prevailing in the gas are mixedwith the gas, and in which the droplets ejected into the gas areelectrically charged and are brought to a potential such that thesurface charge density of the droplets substantially corresponds to anelectric field strength equalling the brakedown field strength of thegas. The droplets thus charged and while carried by the gas are passedwith the gas through a gas flow zone in which the droplets are exposedto evaporation, to cause a self-maintained discharge of the electriccharge of the droplets due to a tendency of increase of field strengthin the vicinity of a droplet as the droplets are diminished duringevaporation. Thereby the charges carried by the droplets are transferredto the dust particles carried by the gas and thereafter the gas carryingthe particles thus charged and any remaining droplets, are passedthrough an electric filter for separating the dust particles thuscharged from the gas. 7

3 Claims, 1 Drawing Figure 1 ELECTROSTATIC GAS-SCRUBBER AND METHOD Inthe art of removing dust from gases, so-called scrubbers are well-known,in which the gas from which dust is to be separated is brought intointimate contact with a liquid, usually water, while, to enhance theinteraction between gas and liquid, the active surface of the liquid isincreased by injecting it into the gas flow as jets or curtains in whichthe liquid is distributed in-the form of droplets.

Furthermore, electro-filters are known for precipitating dust fromgases, in which the dust particles, after havingbeen charged to highvoltageby means of corona discharge, is removed from the gas by aid ofelectrical forces.

Finally, combinations of scrubbers and electro-filters are known, inwhich the gas from which dust is to be separated is brought into contactwith jets of liquid drops brought to high electric potential.

When purifying hot gases by electro-filters there is often a necessityto lower the temperature ofthe gases prior to passing the gas throughthe scrubber or filter, such cooling of the gas being provided for byadmixing cold air or by injecting a cooling liquid, mostly water, inform of droplets.

The present invention relates to utilizing injection of liquid dropletsinto a gas for applying to dust particles carried by the gas an electriccharge, to facilitate the separation of the dust particles in afollowing electrofilter.

When making use of the invention the applying of a charge to the dustparticles to be separated differs in so far from the conventional,method of applying the charge by means of corona discharge fromelectrodes held at high potential, as the dust particles carried by thegas are charged by discharge from charged droplets of a liquid followingthe gas together with the dust particles to be charged, the charge ofthe droplets being ejected from the droplets as they evaporate in thegas while decreasing in size.

The invention can be used also if no demand for cooling the gas ispresent, although a cooling thereof may be as such be allowable. it canused even for purifying comparatively cold gases, although in a somewhatmodified form, in that, for instance, a liquid is heated to atemperature higher than the boiling point thereof at the pressureprevailing in the flow of gas to be purified before ejecting it into thegas and imparted a charge, or by exposing the gas and charged dropletstherein to heat in a zone between the place of injection of chargeddroplets and the following electro-filter.

It is to be emphasized that the injection in accordance with the presentinvention of charged droplets of a liquid which is evaporable under theprevailing conditions is not to be confused with injection of chargeddroplets to separate charged dust particles from the gas, in that, whenusing the invention, dust particles are applieda charge having apolarity corresponding to that of the droplets, the charges of which aretransferred to the dust particles, while in a gas-scrubber of the priorart in which charged droplets of a washing liquid are used to catch dustparticlesgwhether applied a charge or not, the droplets of washingliquid and the dust particles are to be given such polarities thatattraction forces are present between the droplets and thedustparticles.

- As a principle, when using the present invention, the droplets ofevaporable liquid ejected into the gas are supplied with an electriccharge, which, preferably, is so high that the surface charge density ofthe droplets immediately after charging the droplets corresponds to anelectric field strength in the gas adjacent the surface of the dropletequallying or substantially equallying the brakedown field strength. Inair of normal pressure, this field strength is about 30 kilovolts percentimeter. Charging the droplets can, as such, be provided for by anyconventional means for the purpose, such as for instance by arranginginfluence electrodes charged to a high potential around nozzles forinjecting the evaporable liquid. As is well known per se, the efficiencyof the procedure of applying a charge to isolated bodies, in this casethe droplets, by influence has a maximum adjacent the brakedown voltageof the medium surrounding the body, the efficiency decreasing for highervoltage of influence electrodes due to corona discharge losses.

When droplets thus charged are then carried with the gas to be purifiedfrom dust, as the case may be the hot gas, the liquid evaporates fromthe surface of the droplets. Thus, the volume of the droplets issuccessively diminished, the droplets finally possibly entirelydisappearing. Due to the fact that theelectric surface charge densitycan not substantially exceed a value for which the field strengthadjacent a drop corresponds to the brakedown field strength without adischarge into the surrounding gas, the droplets, while yielding vaporto the surrounding gas, ejects an electrical charge as well, this inform of electrons and/or ions, which provide for a charging of the dustparticles present in the gas.

Therewith the total quantity of electricity delivered to the gas can bemade very large and, as a consequence thereof, the ionization of the gasbe high. In practice, this means that the gas can be passed immediatelyto the dust collecting section of an electro-filter of any kind known tothe art. I d

Thus, corona discharge electrodes which are normally necessary inelectro-filters to charge dust particles carried by the gas by means ofionization of the gas can be omitted, implying advantages for dry as.well as for wet electro-filters. In'a conventional electro-filter ofboth types, dust collected in the filter is removed not only from thedust collecting electrodes of the filter, which in filters of the wettype is provided for continuou'sly, but from the ionization electrodesas well, the latter being coveredv by dust due to the extremelynonuniform fields prevailing in the vicinity of the corona dischargeelectrodes of the filter and due to induced bipolic charges supercedingdust repulsion due to coulomb forces. Such. measures are substantiallyeliminated when utilized the invention.

The necessity in conventional electro-filters to use corona dischargeelectrodes to provide for the ionization for charging the dust particlessometimes implies a limitation, namely in respect of the highestpermissible velocity of the gas when passing the apparatus. The chargingof the dust particles in a corona discharge re quires a certain timeperiod, and, consequently, the gas velocity must not be so high that theprevailing time of eliminated in an electro-filter according to theinvention.

According to the invention, an electro-filter, whether of the dry typeor of the wet type, comprises ionization means consisting of at leastone jet of electrically charged droplets of a liquid evaporable underthe conditions prevailing in a gas passage extending between the placeof injecting the droplets and a following filter device.

In a gas scrubber according to the invention the demand for high voltageelectric power'is substantially diminished as compared with conventionalelectrofilters, this due to the fact that influence electrodes tocharge'the jet of droplets to be evaporated in the gas does not as ageneral principle, require any current supply at all, only a supply ofvoltage. The electric energy of and generated in the jet originates fromthe kinetic energy of the jet, this last mentioned energy in part beingtransferred into electro-static energy. With a suitable dimensioning anddesign of means for injecting the jet and electrodes for applying acharge to the droplets, a high efficiency of the energy conversion.process is obtainable, a dimensioning which is as such well known to theart.

The section of the gas scrubber in which dust particles are separatedfrom the gas may be of any conceivable design, having, for instance,charged plates, dry or wetted, for collecting the dust, or being of thekind utilizing charged-droplets of liquid for collecting the dustparticles.

The invention will be more closely described with reference to the soleFIGURE of the accompanying drawing which by way of example schematicallyillustrates an embodiment thereof.

Numeral 1 illustrates part of a duct for flow of a gas, dust particlesof which are to be precipitated, and, also schematically, components ofthe dust separator of importance for understanding the invention. Assuch, the components of a device according to the invention individuallybelong to prior art, wherefore a detailed description thereof will notbe necessary for understanding the invention.

Numeral 2 is the inlet port of the apparatus, after which gas enteringthe apparatus as indicated by an arrow meets jets 3 of droplets of aliquid which is evaporable under the conditions prevailing in theapparatus. The jets are injected into the apparatus by means of nozzles'4 connected to a duct 5 for pressurized liquid. The jets are, asillustrated, injected in a direction opposite to the gas flow, tosomewhat increase the prevailing time of the droplets in the gas beforethe gas then enters a dust separation section of the apparatus. However,the jets may be injected in any direction into'the gas provided thedroplets are so small that soon after injection they adopt substantiallythe same velocity through the duct as the gas.

Each nozzle is surrounded by an influence electrode 6 which is, as knownper se, arranged relative to the jet of liquid ejected from the nozzleso that the jet when subdivided into droplets is exposed to aconcentrated electrostatic field of a strength and polarity depending ofthe potential of the electrodes 6, said electrodes being connected to ahigh-voltage source 7, preferably of positive polarity relative to theenclosure of the grounded enclosure of the apparatus. Droplets of liquidejected from the nozzles 4 then adopt a negative polarity.

Preferably, the potential of electrodes 6 is selected so as to cause noor only insignificant corona discharge from the electrodes, the fieldstrength at the spraying point of the liquid however reaching or onlytoa small degree deviating from the strength where a corona dischargestarts under prevailing conditions. Droplets ejected from the nozzlethen acquire the highest possible surface charge, namely, a surfacecharge density of a magnitude that the gas in the immediate vicinity ofthe droplets is exposed to an electric field originating from the chargeof the droplets in the order of kilovolts per centimeter, which is the.value leading to electric breakdown in normal air of atmosphericpressure.

When the gas to be purified, in most cases in practice a comparativelyhot and dry gas, meets the jets of charged droplets, the droplets startevaporating, and evaporate, entirely or in part, while being carriedalong the duct 1 together with the gas. The electric charge carried bythe droplets is then continuously liberated as I the radius of thedroplets is diminished, the liberated charge appearing in the gas inform of ions and charges landing on dust particles.

The dust particles carried by the gas through the duct then passcollecting electrodes 9 of the electro-filter proper of the apparatus,to which they are attracted, and adhered. Electrodes 8, located betweenpairs of I electrodes 9 and collected to ground provide, together withelectrodes 9, for the electric field causing the precipitation of thecharged dust particles carried with the gas, as well known per se. Asillustrated, electrodes 9 are connected to the same voltage source asthe influence electrodes 6. Obviously, electrodes 9 could as well beconnected to othervoltage source.

Gas purified from dust particles then leaves the apparatus as indicatedby the arrow through an outlet port 10 of the apparatus.

The electric power, represented by the surface charge of the evaporatingdroplets is, in the embodiment of illustrated in the drawing, generatedby the flow of liquid by a mechanical charge separatiomthat is, byconverting the kinetic energy of the liquid into electric energy. Underthe conditions described, the corresponding transport of charges,expressed in terms of electric current, and under the presumption thatthe diameter. of the droplets of liquid'is in the order of a size 0.1millimeter whenleaving the spraying zone, is for a flow of liquid of 1liter per second of the order of a size of 1.6 mA or 10 elementarycharges 'per second, the larger part thereof being available forcharging dust particles.

Should an apparatus for precipitating dust from gases be intended forbeing used for purifying gases which are too cold to cause a substantialevaporation of the liquid forming the droplets, the charge of which istransferred to dust particles in the apparatus, the apparatus itselfcan, without deviation from the idea of the invention, be provided'withmeans for supplying heat to the droplets to cause evaporation thereof inthe range between injection of the droplets and the location of theelectro-filter proper in which the dust is removed. Such means areschematically illustrated in the drawing by electric heat radiators ll,electrically connected to a suitable power source, and, as representingan auxiliary equipment necessary under rare circumstances only, shownwith dashed lines.

While this invention has been described with respect to specificexamples thereof, it should not be construed as being limited thereto.Various modifications and substitutions will be obvious to everyoneskilled in the art and can be made without departing from the scope ofthis invention.

What I claim is:

l. A method of removing dust from a gas in which dust particles to beremoved from the gas are supplied with an electric charge before the gasis passed through an electro-filter for removing the dust particles thuscharged from the gas, said method comprising applying to said dustparticles an electrical charge by the steps including: dischargingdroplets of a liquid which is evaporable under the conditions prevailingin the gas into the gas at a location upstream of said electro-filter,applying an electrical charge to said droplets of liquid, heating saiddroplets to effect evaporation thereof in a gas flow passage extendingbetween a location where the droplets have had said electric chargeapplied thereto and said electro-filter to cause emission of the chargesfrom said droplets, while the droplets evaporate, and transfer of saidcharges to the dust particles; and thereafter passing the gas throughsaid electrofilter where the thus charged dust particles are re moved.

-2. An apparatus for removing dust from a gas, said apparatus comprisinga duct having an inlet and an outlet, an electro-filter for separatingelectrically charged dust particles from the gas, said electro-filterbeing located in said duct, means for applying an electrical charge tothe dust particles in said duct carried by the gas, the latter saidmeans being located upstream of said electro-filter and comprising meansfor injecting droplets of a liquid into the gas, means for applying anelectrical charge to said droplets, and means for supplying heat to thedroplets in said duct between the location of said means for applyingthe electrical charge to said droplets and said electro-filter to causeevaporation of said droplets and transfer of the charge to said dustparticles before the gas passes into the electrofilter, said means forinjecting the droplets of liquid comprising a nozzle facing said inletfor injecting the droplets in counter-current with the supplied gas,said means for applying an electrical charge including an electrodeadjacent said nozzle.

3. Apparatus as claimed in claim 2 wherein said means for supplying heatto evaporate the droplets comprises electrical heat radiators disposedin said duct between the nozzle and the electro-filter.

2. An apparatus for removing dust from a gas, said apparatus comprisinga duct having an inlet and an outlet, an electro-filter for separatingelectrically charged dust particles from the gas, said electro-filterbeing located in said duct, means for applying an electrical charge tothe dust particles in said duct carried by the gas, the latter saidmeans being located upstream of said electro-filter and comprising meansfor injecting droplets of a liquid into the gas, means for applying anelectrical charge to said droplets, and means for supplying heat to thedroplets in said duct between the location of said means for applyingthe electrical charge to said droplets and said electro-filter to causeevaporation of said droplets and transfer of the charge to said dustparticles before the gas passes into the electro-filter, said means forinjecting the droplets of liquid comprising a nozzle facing said inletfor injecting the droplets in counter-current with the supplied gas,said means for applying an electrical charge including an electrodeadjacent said nozzle.
 3. Apparatus as claimed in claim 2 wherein saidmeans for supplying heat to evaporate the droplets comprises electricalheat radiators disposed in said duct between the nozzle and theelectro-filter.